Mary Lizzie Quinn - born 3 November 1898 - has not been completely ruled out, but was thought by older people in the district not to be the person in question.
Elizabeth told the priest she was a minor, and a check in the baptismal register would have shown that Mary Lizzie Quinn was not a minor, but almost 22. The parish priest, Fr James Loughran, invariably (though not always) entered marriage details of both parties above their baptismal entries: in this instance he entered James's, but there is no corresponding entry above the baptism of Mary Lizzie Quinn, born 1898.
There is also the consideration that Patrick Quinn was a farmer, not a labourer, and the Land Valuation Book shows that he still owned his farm in 1929. It appears that this Quinn family sold their farm and moved away from Ardboe in the 1930s.
In the baptismal entries of the four O'Neill children, the mother is always named as Lizzie Quinn, never as Mary Lizzie.
In the 1901 and 1911 census returns for the family of Patrick Quinn, farmer, their daughter's name is entered as Mary Lizzie.
These little pointers do not discount her, but increase the likelihood that the wife of James O'Neill was another person....